Problem: Solve for $r$ : $-15 + r = -23$
Explanation: Add $15$ to both sides: $ \begin{eqnarray} \\ -15 + r &=& -23 \\ \\ {+15} && {+15} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{-15 + r} &\hphantom{=}& \hphantom{-23} \\ r &=& -23 {+ 15} \\ \end{eqnarray} $ Simplify. $ r = -8$